1. Field of the Invention
The present invention relates to a driving apparatus to be applied to a yoke-rotational stepping motor.
2. Related Background Art
Conventionally, a stepping motor has been widely used for driving sources of various mechanisms. As a first conventional example of the stepping motor, a stepping motor is proposed in which the diameter centering around a rotation axis is decreased and the output is raised (for example, refer to Japanese Patent Application Laid-Open No. H09-331666 (U.S. Pat. No. 5,831,356)).
FIG. 22 is an exploded perspective view showing a component of a stepping motor of a first conventional example. FIG. 23 is a sectional view showing the structure of an already-assembled stepping motor.
In FIGS. 22 and 23, the stepping motor is provided with a magnet 301, first coil 302, second coil 303, first stator 304, second stator 305, output shaft 306 and connecting ring 307. The magnet 301 is formed like a cylindrical shape, divided into four parts in the circumferential direction and alternately magnetized to different polarities. The first stator 304 is magnetically excited by the first coil 302. The second stator 305 is magnetically excited by the second coil 303.
The stepping motor having the above configuration changes polarities of the first outside magnetic pole portions 304A and 304B, first inside magnetic pole portions 304C and 304D, second outside magnetic pole portions 305A and 305B and second inside magnetic pole portions 305C and 305D by changing the electrifying directions for the first coil 302 and second coil 303. Thereby, a rotor constituted of the magnet 301 and output shaft 306 is rotated. In this case, the magnet 301 is attached to the output shaft 306 with an adhesive or the like.
In the case of this stepping motor, a magnetic flux generated by electrifying the first coil 302 and second coil 303 flows to an opposite inside magnetic pole portion from an outside magnetic pole portion or opposite outside magnetic pole portion from an inside magnetic pole portion and efficiently acts on the magnet 301 located between the outside magnetic pole portion and the inside magnetic pole portion. Moreover, it is possible to set the distance between the outside magnetic pole portion and the inside magnetic pole portion to approximately the thickness of the cylindrical-shaped magnet 301. Therefore, it is possible to decrease the resistance of a magnetic circuit constituted of an outside magnetic pole and inside magnetic pole. Therefore, it is possible to generate more magnetic flux with a small current as the resistance of a magnetic circuit decreases and an output of the stepping motor is improved.
Moreover, as a second conventional example of a stepping motor, a hollow cylindrical-shaped stepping motor is proposed (for example, refer to Japanese Patent Application Laid-Open No. 2002-51526 (U.S. Pat. No. 6,798,093 and U.S. Pat. No. 6,800,970). When mounting the stepping motor of this type on a camera, the motor is set so that it becomes parallel with the optical axis in the lens barrel of the camera to set an aperture blades, shutter, lens and the like to the inside-diameter portion of the motor. Thereby, it is possible to decrease the diameter of the lens barrel of the camera.
FIG. 24 is an exploded perspective view showing a component of the stepping motor of the second conventional example. FIG. 25 is an illustration showing a structure of an already-assembled stepping motor.
In FIGS. 24 and 25, the stepping motor is provided with a rotor 401, first coil 402, second coil 404, first stator 418, second stator 419 and connecting ring 420. The rotor 401 is inserted into the inside-diameter portion of the connecting ring 420 and rotatably held by convex portions 420a and 420i of the connecting ring. Moreover, by setting a pin 401t to the rotor 401, an output for moving the aperture blades or lens barrel is fetched through the pin 401t. Reference numerals 418a to 418h denote magnetic-pole teeth.
However, conventional stepping motors described in the above Japanese Patent Application Laid-Open Nos. H09-331666 (U.S. Pat. No. 5,831,356) and 2002-51526 (U.S. Pat. No. 6,798,903 and U.S. Pat. No. 6,800,970) respectively constitute a rotor by attaching a magnet to an output shaft or output pin with an adhesive or the like.
Therefore, there is a problem that the quality of a stepping motor is not stabilized due to the rotor assembling accuracy, overflow of an adhesive or imperfect adhesion. Particularly, when decreasing the diameter of the stepping motor, overflowed adhesive occupies a relatively large volume and interrupts stable rotation of a rotor.
Moreover, to achieve high speed of and improvement of a response speed of a stepping motor, it is necessary to decrease the moment of inertia of a rotor. However, it is difficult to decrease a conventional stepping motor in weight and decrease of the moment of inertia of the rotor is restricted.